1. Field of the Invention
The present invention relates to a transparent laminated product having electrical conductivity, infrared reflectivity and electromagnetic wave shielding effect.
2. Discussion of Background
Heretofore, it has been in practice to form a layer of indium oxide or tin oxide on a glass or plastic substrate and to use the product as a transparent conductive substrate. However, the specific resistance of the transparent conductive layer of this type was at a level of 5.times.10.sup.-4 .OMEGA..cm when formed at room temperature, and in order to obtain a surface resistance of not higher than 10 .OMEGA./sq., it used to be required to have a layer thickness of at least 5,000 .ANG.. Further, the transparent conductive layer of this type had a refractive index of about 2.0, and when formed on a glass substrate having a refractive index of e.g. 1.5, it had a reflectance as high as about 25%, whereby the surface glare was strong, such being undesirable from the viewpoint of outer appearance.
It has been reported that the specific resistance of the transparent conductive layer of this type can be reduced to a level of 1.times.10.sup.-4 .OMEGA..cm by forming such a transparent conductive layer on a substrate at a high temperature. However, it is undesirable to heat the substrate from the viewpoint of the production, since such operation adds to the cost.
There are two types of infrared reflecting glass. Namely, the first type is a so-called solar control which is used primarily for the purpose of reducing an air-cooling load, and the second type is a so-called heat mirror which is used primarily for the purpose of reducing a heating load. The minimum characteristics required for the second type are a high transmission at the visible region and a sufficiently high reflectance at the infrared region. However, if the reflectance at the near infrared region can be increased, it will also have a function as a solar control, such being preferred. The following three types of coating have been known for the infrared reflecting glass of this type:
(1) Thin layer of a metal having a thickness of the about 100 .ANG.. PA0 (2) Doped oxide semiconductor layer PA0 (3) 3-Layered coating of dielectric layer/metal layer/dielectric layer. PA0 (a) In order to obtain a high transmission at the visible region and a sufficiently high reflectance at the infrared region, type (3) is the best. However, it is still inadequate. PA0 (b) The color of reflection is limited to a color of violet type. PA0 (c) The rising of reflectance at the near infrared region is not sharp.
Specifically, as type (1), a thin layer of Au, Ag or Cu is used, and as type (2), a layer of SnO.sub.2 or In.sub.2 O.sub.3 having a thickness of at least 5,000 .ANG. is used. As type (3), a structure in which a silver layer is sandwiched between dielectric layers, is disclosed in Japanese Examined Patent Publication No. 6315/1972.
Among them, type (1) has a drawback such that in order to obtain a sufficiently high reflectance at the infrared region, the metal layer is required to be thick, whereby the transmittance at the visible region will be low. Type (2) has drawbacks such that in order to obtain a sufficiently high reflectance at the infrared region, the thickness of the layer is required to be as thick as at least 5,000 .ANG., and the reflectance at the near infrared region can not be improved.
Whereas, type (3) is advantageous in that the dielectric layers having a metal layer interposed therebetween serve as a reflection preventing layer, whereby a sufficiently high reflectance at the infrared region and a high transmission at the visible region can be obtained with an overall layer thickness of not higher than 1,000 .ANG., and thus this type is widely used. However, even with this type, the thickness of the intermediate metal layer is required to be not higher than 200 .ANG., preferably not higher than 150 .ANG., in order to obtain a high level of transmittance at the visible region, whereby the reflectance at the infrared region will be about 95% at best, hence the emittance will be about 5%. Further, the reflectance at the near infrared region can not be improved, and the reflectance of the solar energy will be not higher than about 25%.
In order to obtain a sufficiently high visible ray transmission and at the same time to control the visible ray reflectance to a level of a usual transparent glass, the silver layer thickness was limited to a level of about 120 .ANG., and the surface resistance was thereby about 12 .OMEGA./sq. This is because the specific resistance of silver in the form of a thin layer is greater than the value of the bulk. Further, as will be given as a Comparative Example hereinafter, the spectral reflectivity will be in a U-form at the visible region, and the reflection color will be restricted to a color of violet type, whereby variation in the color will be limited, which is a serious drawback from the ornamental point of view.
In summary, the conventional techniques had the following drawbacks: